Combined humidifier and preheater of air for combustion apparatus



Feb. 1, 1955 H. s. HILL COMBINED HUMIDIFIER AND PREHEATER OF AIR FOR COMBUSTION APPARATUS 3 Sheets-Sheet 1 Filed Nov.

BY 22. M J

flfi orire ys.

INVENTOR. 012M204 iaez Feb. 1, 1955 H. s. |-m 1 2,700,945

COMBINED HUMIDIFIER AND PREHEATER OF AIR FOR COMBUSTION APPARATUS Filed Nov. 17, 1950 3 Sheets-Sheet 3 0/ INIfENTOR.

United States Patent COMBINED HUMIDIFIER AND PREHEATER OF AIR FOR COMBUSTION APPARATUS Harold S. Hill, Lockport, N. Y.

Application November 17, 1950, Serial No. 196,155

2 Claims. (Cl. 110-56) This invention relates to a combined humidifier and fuel or air preheater for combustion apparatus and is more particularly described in conjunction with liquid fuels derived from petroleum, shale and coal and used in oil burning furnaces, diesel and gasoline engines and the like although features of the invention are applicable to combustion apparatus using other fuels whether in liquid or gaseous state.

This application is a continuation in part of my copending applications Serial No. 120,908, filed October 11, 1949, now Patent No. 2,592,568 issued April 15, 1952, for Method and Apparatus for Preheating Liquid Fuel and Serial No. 157,393, filed April 21, 1950, now

Patent No. 2,625,211 issued January 13, 1953, for Oil and Air Preheaters for Oil Burners.

In common with my said copending applications important objects of the present invention are, by preheating of both the fuel and air supporting combustion, to reduce the viscosity of the fuel oil to bring heavy fuels within the range of utility as a liquid fuel; to facilitate the start of combustion by supplying fuel and air atva higher temperature on starting than during continued op eration; to move atomization of the fuel oil, particularly on starting, and insure more intimate admixture of the fuel and air thereby to require less air and provide improved combustion; to effect such preheating of the fuel and air during both off and on periods of operation in such manner as to avoid scorching of the fuel oil and also to slow down the application of heat and thereby level oil the on and ofi periods of operation and maintain more uniform liquid fuel and air preheating conditions; which can be used to vaporize liquid fuel if desired; which is under simple thermostat control and not subject to hunting or erratic action; and which is economical in consumption of electrical or other energy.

An important specific object of the present invention is to humidify the air supplied to the combustion apparatus being served and thereby to improve the combustion reaction and provide increased efficiency of combustion as Well as freedom from soot due to incomplete combustion.

Another specific object is to provide such humidification in the form of steam, preferably superheated, to obtain the maximum effect from the humidification.

Another specific object is to provide a common heating element or elements for preheating the oil and/or air and for generating the steam.

Another specific object is to provide such humidification in which the heat required during running conditions can be in whole or part supplied from the heat, often wasted, generated by the apparatus being served.

Another object is to preheat the air supplied for combustion by the admixture therewith of live steam.

Another object is to provide a slight time delay on starting before humidification is effected.

Other objects and advantages of the invention will appear from the following description and drawings in which:

Fig. 1 is a side elevational view of an oil burner having a combined fuel and air preheater and humidifier embodying the present invention.

Fig. 2 is an enlarged vertical longitudinal section through the casing for the combined fuel and air preheater and humidifier embodying the present invention, the preheater and humidifier being shown in elevation.

Fig. 3 is a further enlarged vertical longitudinal secice tional view similar to Fig. 2 but showing the preheater in section.

Fig. 4 is a bottom plan view of the preheater and humidifier embodying the present invention, parts being broken away.

Fig. 5 is a fragmentary vertical transverse sectional view taken generally on line 55, Fig. 3

Fig. 6 is a fragmentary vertical transverse sectional view taken on line 66, Fig. 2.

Fig. 7 is a wiring diagram of the electrical control circuit for the combined air and fuel preheater and humidifier forming the subject of the present invention.

The invention is shown in conjunction with a conventional oil burner having a fan housing 10 which comprises a scroll 11, this scroll 11 discharging into a horizontal tubular tangential outlet 12. This tangential tubular outlet 12 is provided with a discharge nozzle 13 for the burning fuel and air and is shown as extending horizontally through the wall 14 of a combustion chamber, such as the wall of a furnace. The fan housing 10 can be supported in any suitable manner, as by a leg 15, and the scroll 11 of the fan housing is shown as having one vertical side wall 16 on which a fan motor 18 is suitably mounted, this motor having a drive shaft 19 extending through the fan scroll. Within the fan scroll 11, a fan wheel 20 is fast to the fan shaft 19, this fan wheel 20 being of the usual construction to draw air through the inlet opening or eye 21 of the fan scroll and to discharge it through the tangential tubular outlet 12 and nozzle 13.

Around the inlet or eye 21 of the fan scroll, the fan scroll is formed to provide an outwardly projecting rim 23 to which is removably secured, as by screws 24, a hood 25 which forms a continuation of the rim 23 and which is generally rectangular in horizontal section. The bottom of this hood 25 is opened, as indicated at 26, and in the opening 26 is fitted the outlet neck 28 of an air inlet shell indicated generally at 29. This air inlet shell is made of sheet metal and is of generally rectangular form in horizontal section, the open bottom 30 of this shell being spaced from the floor, and the bottom portion of this air inlet shell being enlarged to receive a combined air and fuel preheater and humidifier indicated generally at 31. All of the walls of the air inlet shell 29 are spaced from the corresponding walls of the preheater and humidifier 31 so that air is free to pass through the vertical space 32 between the preheater and humidifier 31 and the air inlet shell 29 up into the hood 25 and thence into the inlet opening or eye 21 of the fan scroll 11 to be thence discharged by the fan wheel 20 through the tubular tangential outlet 12 and nozzle 13 into the combustion chamber formed by the wall 14 of the furnace.

The preheater and humidifier 31 is preferably con structed in the manner shown in my said copending application Serial No. 120,908 and to this end is shown as comprising a cast metal casing through which the liquid fuel for the burner fiows and which is shown as comprising a relatively thick bottom wall 35, relatively thick side walls 36 rising from the margin of this bottom wall, and a relatively thin top wall 38 one end of which rises to provide a thin walled dome 39. The casing is formed to provide a relatively thick integral darn 40 which ex tends transversely across the interior of the casing and divides the same into an inlet chamber 41 and an outlet chamber 42, the latter being disposed under the thin walled dome 39.

As described in greater detail in my said copending application, Serial No. 120,908, the relatively thick bottom wall is provided with upwardly projecting transversed ribs 43 which accelerate the transfer of heat from this bottom wall 35 to the oil flowing through the chambers 41 and 42 of the casing, and the dome 39 is also provided with a plurality of integral, depending, transverse ribs 44 which accelerate the transfer of heat from the oil within the dome 39 to the thin wall of this dome, thereby to condense the lighter and more volatile fractions of the oil which segregate in this dome. To further accelerate this condensation, the dome 39 is provided with external fins 45. These fins 45 also serve to transfer heat to the surrounding air and to further accelerate such transfer the top wall 38 and the side walls 36 are also preferably provided with external fins 46 and 48, respectively. Each of the chambers 41 and 42 is also filled with a heat conductive foraminous material, such as folded screening 49. This acts to impede direct or short circuit flow of any part of the oil through the two chambers; to distribute heat more uniformly throughout the fuel both when moving through the chambers and when quiescent; and to eliminate convection movement of the oil in either chamber when heated in a quiescent condition.

To distill the more volatile fraction of the oil and to cause this more volatile fraction to rise and segregate in the dome 39 of the casing, the lower part of the body of fuel Within the chambers 41 and 42 of the casing is heated to a temperature above the initial flash point of the liquid fuel passing through the casing. To this end, two electrical heating elements 50 and 51 are shown as arranged immediately below the relatively thick bottom 35 of the casing 31 and are disposed in a chamber 52 formed by the relatively thick bottom 35, a skirt 53, and a removable bottom plate 54 which is removably supported within the skirt 53 as by screws 55, as best shown in Figs. 3 and 4. In addition, the casing 31 is preferably provided with three legs 56 which form downward continuations of the skirt53. To permit of the escape of heat directly from the elements 50 and 51 to the space 32 through which air is supplied for combustion, this skirt 53 can be notched or slitted at intervals as indicated at 57.

While one heating element could be used in lieu of the two heating elements 50 and 51, two heating elements have been found more satisfactory in obtaining the various wattage desired in different preheaters with a minimum number of heating elements. Thus, with a number of heating elements having different wattages, with different permutations of these heating elements it is possible to obtain a wide range of effective heating. These heating elements are preferably arranged in parallel, as shown in the wiring diagram, Fig. 7, and these heating elements are in series with a thermostat 60, the sensitive parts of which are disposed within an aperture 61 in the relatively thick dam 40 and hence influenced by the temperature of the oil within the chambers 41, 42 as transmitted through the thick wall of this dam. This thermostat is set to cut into and out of operation so as to maintain the temperature of the oil contained within the casing 31 at a point slightly above the initial flash point of the oil so as to distill the more volatile fraction thereof, this distilled fraction migrating upwardly into the dome 39 where it encounters the relatively cool wall of this dome so as to become condensed and provide a more volatile body of fuel oil in this dome. As described in greater detail in my said copending application, Serial No. 102,908, this more volatile fraction is available for use in starting the oil burner, the oil originally supplied to the burner not only being warmed but also being of a higher degree of volatility than the overall composition of oil supplied to the burner.

Oil is supplied to the casing 31 by a pump 62 having an inlet line 63 which extends through one wall of the hood 25 and connects with the usual supply tank (not shown). The pump 62 is arranged within the hood 25 and is driven by the drive shaft 19 of the motor and which also carries the fan wheel 20. To support the pump 62, a bracket 64 is suitably secured to the annular rim 23 of the fan scroll 11, the pump being suspended from this bracket. The outlet of the pump 62 connects with a line 65 leading to the inlet chamber 41 of the casing 31.

The oil from the casing 31 is discharged from the dome 39 through an outlet line 66, this outlet line extending through the adjacent walls of the air inlet shell 29 and horizontal tubular discharge 12 of the fan scroll 11 and thence coaxially along this tubular discharge 12 to the discharge nozzle 13 thereof. Within this nozzle 13 the usual fuel nozzle 68 is mounted on the oil pipe 66. this nozzle discharging toward the open end of the nozzle 13 of the tubular air discharge 12.

In the operation of the preheater for the fuel and air supplied to the combustion chamber, as above described, it is desirable to have a controlled and balanced heating of the air as well as of the fuel supplied to the combustion chamber. To this end a third electrical heating coil 70 is provided, this heating coil. 70 enCircling the side wall 30 of the casing 31 so as to be disposed in the space 32 between this casing 31 and the surrounding air inlet shell 29. This heating coil 70 is also preferably finned, as indicated at 71, so as to provide a higher rate of heat transfer to the air flowing up this space 32. This electrical heating element 70 can be supported on the casing 31 in any suitable manner as by the provision of small brackets 72 at suitable intervals for this purpose. This electrical heating element 70 is preferably in parallel with the other two heating elements and 51 and hence is under control of the thermostat which is responsive to the temperature of the oil flowing through the casin 31.

While the fuel burning apparatus is in operation it is desirable to supplement the heat supplied by the electrical heating elements 50, 51 and by the waste heat generated by the combustion apparatus. This waste heat can be in the form of the exhaust gases generated by the apparatus or, in the case of diesel engines and the like, in the form of the jacket water used to cool the engine. To utilize this waste heat, a U-shaped pipe 73 is shown as horizontally disposed within the chamber 52 under the casing 31 and as having its opposite ends 74 and 75 projecting outwardly and adapted to be utilized as the inlet and the outlet pipes for the auxiliary heating mediums, such as exhaust gas or jacket water. It is also desirable to preheat the air supplied to the combustion apparatus through heat derived from the exhaust gas or jacket water of the apparatus being served and to this end a finned tube 76 is shown as having one end connected to to the inlet 74 for the U-shaped pipe 73 and having its opposite end connected to the outlet 75 thereof. This tube 76 is arranged in the space 32 between the casing 31 and the surrounding air inlet shell 29 and embraces the casing 31. In order to improve the heat transfer characteristics of this tube, it is preferably provided with fins as indicated at 78.

In some applications it may be desirable to prevent overheating of the oil by the exhaust gas or jacket water supplied through the coil or pipe 73. To avoid such overheating a solenoid valve 79 can be provided in the inlet 74 to this coil or pipe as best shown in Fig. 4. This solenoid valve can be under thermostat control as by arranging it across the heating elements 50, 51 and 70 so as to be under control of the thermostat 60, as best shown in Fig. 7.

An important feature of the present invention resides in the means for humidifying the air supplied to the combustion chamber, preferably by live, superheated steam. For this purpose, as best shown in Fig. 5, a small water inlet tube 80 is provided, the water flowing through this tube being under control of a normally closed on and off solenoid valve 81 and discharging into a vertical bore 82 provided through the cover or top wall 38 and body of the casing 31. As best shown in Fig. 7, this solenoid valve is in series with the motor 18, both being under control of a thermostat 83, such as a room thermostat if the apparatus is being used for space heating. The inlet end 84 of a water coil 85, as best shown in Figs. 5 and 4, is secured in the lower end of the bore 82. This water coil 85 is arranged in the chamber 52 provided by the bottom plate 54 for the casing 31 and the convolutions of this water coil are preferably arranged in intimate contact with the two electrical heating elements 50 and 51. The outlet 86 of this water coil 85 projects upwardly and is secured in a second bore 88 extending vertically through the body and cover or top wall 38 of the casing 31, the upper end of this bore discharging into a cup-shaped upward extension 89 and the cover or top wall 38. The top of this cup-shaped projection 89 can be covered, as by a perforated cap 90, and this cupshaped projection 89 can be drained by a drain line 91 (Fig. 3). In its passage through these conduits to the cup-shaped projection 89 the water is vaporized into live steam and is discharged as live steam through the perforations of this cap into the passing air stream. The amount of water is under control of a manual needle valve 93 as shown in Figures 5 and 7. The steam so supplied to the air traveling to the combustion apparatus not only humidifies but also preheats this air. It will I also be noted that there is necessarily a slight delay after start of combustion before steam is generated and supplied to the stream of air which supports combustion. This is desirable in preventing possible condensation and other difficulties which might develop if humidification were instantaneous on starting.

In the operation of the apparatus as above described, the motor 18 is automatically cut into and out of operation in response to the demands for heat or power, such as through the operation of the thermostat 83 which, in space heating, would be a room thermostat. During oflt periods of operation no fuel is supplied by the pump 62 and hence the body of liquid fuel in the casing 31 becomes a stagnant or quiescent body. The thermostat 60 which is responsive to the temperature of the oil contained within the casing 31 is set a few degrees above the initial flash point of the fuel being used and preferably, though not necessarily, below the fire point or lowest temperature at which heating oil gives off vapor that supports continuous combustion, this fire point being usually from ten to fifteen degrees F. higher than the initial flash point. Thus, if the initial flash point of the liquid fuel being used is, say, 148 F., and the tire point is, say, 160 F, the thermostat 60 would be set to cut into operation at 150 F., and cut out of operation at 153 F. This setting of the thermostat is readily accomplished by the operator who places the apparatus in operation, the operator knowing the flash point and fire point from data supplied to him or determining the flash point of the liquid fuel for himself and then taking the temperature of the dam 40 a thermometer opening 92 being provided for this purpose. With such temperature the operator can readily set the thermostat 60 to cut into and out of operation at the temperature indicated.

With the thermostat 60 so set, it calls for heat during on periods of the burner operation, and accordingiy preliminary to the off period of operation, the heating elements 50 and 51 are energized and heat is being supplied by these elements to the relatively thick bottom wall 35 of the casing 31. This energization of the electric heating elements 50 and 51 continues until the temperature of the control dam 40 builds up to the assumed temperature of 153 F., the quiescent body of liquid fuel in the bottom of the chambers 41 and 42 also being heated to approximately this temperature. It will be noted that this heat is supplied at the bottom of the quiescent body of liquid fuel by the thick bottom wall 31, and, through conduction through the metal, at the sides of the body of liquid fuel and from the thick control darn 40 and relatively thick side walls 36 of the casing 31.

As each particle of the liquid fuel is heated above its flash point, the lighter or more volatile fraction vaporizes and rises in the quiescent body of liquid fuel. On en countering the cooler fuel, these vaporized particles, of course, recondense, but since heat is supplied from the sides and bottom of the quiescent body of fuel, the net effect of the vaporization and recondensing of the light ends of the liquid fuel is a migration of the light ends to the top of the body of liquid fuel. Since the top of the oil chamber is provided by the dome 39, these lighter ends migrate up into the top of this dome. Since this dome 39 is relatively thin walled, and also is provided with the fins 45 so as to rapidly dissipate heat to the surrounding air, after entering the dome 39 and contacting its walls, the light ends of the fuel oil are cooled below their flash point and thus condense as a segregated body of light ends within the dome 39. These light ends also ascend the tube 66 to replace the heavier fuel oil therein and hence these light ends are immediately available on starting.

The migration of the lighter ends from the inlet chamber 41 is impeded by the transverse rib 44 arranged directly over the dam 40 and under which any particle of fuel oil must pass in close proximity to the heated dam 40 before it can escape to the dome 39. This provides independent distillation action in the two chambers 41 and 42, only the substantially completely distilled lighter ends being permitted to escape from the chamber 41 into the chamber 42.

Accordingly during off periods of demand, mild but rapid distillation action takes place which effects a segregation of the light ends or more volatile fraction in the dome 39 and line 66. Therefore, when the furnace or other apparatus served goes into operation, the fuel initially supplied is not only heated to a relatively high temperature but is also more highly volatile than the fuel supplied to the preheater by the pump 62. Accordingly, when this fuel is discharged through the outlet line 66 from the casing 31 and through the fuel nozzle 68, ignition of this initially supplied fuel at the start of each on period is materially expedited and proper combustion of the fuel on each start is assured.

After the starting period the combustion of the less volatile fuel supplied at a lower temperature presents no problem, proper economy in fact demanding that the burner be set to operate with the fuel of average volatility and a lower preheated temperature. Accordingly, the capacity of the two heating elements 50 and 51 is deliberately selected so as to heat the liquid fuel flowing through the preheater on continued operation of the fuel burner to only a fraction of setting of the thermostat 60. This is for the reason that during a continued on period the combustion of the fuel in the apparatus supplies part of the heat for eflicient operation so that preheating to a lower temperature is desired. It will also be noted that the transition from the high initial temperature of fuel oil at the start of each on period to the relatively low temperature running is gradual, this being due to the heat in the body of liquid fuel contained. in the chambers 41 and 42, and in the relatively thick bottom wall 35, relatively thick side walls 36 and also the relatively thick control dam 40 of the preheater casing 31.

As long as the on period lasts preheated liquid fuel is supplied by the preheater and when the demand is satisfied, the motor 18 is deenergized, as by the room thermostat at 83, thereby to deenergize the fuel pump 62 and stop the flow of liquid fuel through the preheater casing 31. Accordingly, the now quiescent body of liquid fuel in this preheater casing 31 is again heated to a few degrees above its flash point to again segregate the lighter ends in the dome 39 for the start of another on period.

During off periods of operation, as indicated, a part of the heat generated by the electrical heating elements 511 and 51 is dissipated by the dome 39 and casing 31 to the surrounding atmosphere in effecting partial distillation and segregation of the lighter ends of the fuel oil. Since the body 31 of the preheater is surrounded by the air inlet shell 29, this heated air is trapped and rises in the upper outlet neck 28 of this shell into the hood 25 and scroll 11 of the fan housing. Accordingly, at the start of operation heated air is supplied to support the combustion of the preheated lighter ends being discharged from the fuel nozzle 68, this preheated air being drawn through the air inlet shell 29, hood 25 and scroll 11 by the operative fan Wheel 20 and being discharged through the tangential tubular outlet 12 to mix with the preheated lighter ends being discharged by the fuel nozzle 68 into the furnace 14.

During continued on periods of operation, while it is desirable that the oil supplied to the nozzle 63 be heated to a lesser degree than at the start of operation, some preheating of both the air and also the fuel is desirable. This preheat can in part be supplied by the waste heat of the apparatus being served and to this end the products of combustion or the jacket water from the apparatus being served can be circulated through the coils 73 and 76. The circulation of this normally waste heating medium through the coil '70 Within the chamber 52 serves to augment the heating effect of the two electrical elements 50 and 51 and to preheat the oil now flowing through the two chambers 41 and 42 of the preheater casing. The flow of this waste heating medium through the thin coil 76 in the space between the preheater casing 31 and the surrounding air inlet hood 29 serves to heat the air being supplied to the combustion apparatus. This heating of the air supplied to the combustion apparatus during on periods of operation can be augmented by the finned electrical heating element 70 which is also arranged in this space 32. An important feature of the present invention is the ability to be able to balance the amount of heat separately added to the fuel and air supplied to the apparatus being served during both on and off periods of operation. This is readily accomplished by the use of electrical heating elements of the desired wattage to obtain this desired balance. It will be noted that while the finned heating element 70 is energized during off periods of operation and up to the setting of the thermostat 60, this heat is not wasted since it serves to heat the casing 31 through its close proximity therewith and hence add to the heating effect of the two electrical heating elements 50 and 51.

This balanced preheating of both the fuel oil and also of the air, as well as balanced humidification of the air during continued periods of operation is an important feature of the present invention. So far as preheating of the fuel is concerned during continuation of the operation of the burner, the principal need for preheating is to bring the viscosity of the relatively heavy fuels used within the range of utility as a liquid fuel. within the burner. This reduction in the viscosity of the fuel oil is principally accomplished by the preheating of the fuel oil, the viscosity of the fuel oil being an inverse function of its temperature, the viscosity falling as the temperature rises. Better combustion is obtained through preheating both the fuel and the air, because combustion is a chemical reaction which is accelerated by an increase of the temperature of the reacting substances.

Balanced preheating of both the air and also the fuel oil has also been found to provide improved atomization of the fuel oil in the preheated air. Atomization is a function of viscosity and improved atomization has been found to result from preheating both the fuel oil and the air so as to reduce the viscosity of the oil. Improved atomization in turn provides more intimate admixture of the fuel oil with air, and hence less air is required and also less fuel oil is required because of the improved combustion.

The principal feature of the present invention resides in combining with the above described functions humidification of the air supplied to the burner, preferably in the form of live and superheated steam. Thus when the thermostat 83 (Fig. 7) or other suitable instrument closes in response to a demand for heat or power, in addition to the fan motor 18 being energized the normally closed solenoid valve 81 is also energized so as to permit the flow of water from the supply line 80 to the bore 82 (Fig. in the preheater 31. This solenoid valve remains open and permits the flow of water as long as the fan motor 18 operates, the amount of water fed being under control of the needle valve 93 shown in Fig. 7. It will be noted that when the room thermostat 83 or other instrument is satisfied, the solenoid valve 81 closes simultaneously with the deenergizing of the fan motor 18.

Accordingly water is supplied from the supply line 80 (Fig. 5) through the open solenoid valve 81 into the bore 82 and thence into the inlet 84 of the water coil 85 and out through the outlet 86 and the bore 88. Since during off periods of demand the electrical heating elements 50 and 51 are energized through the thermostat 60 to maintain the temperature of the fuel in the preheater casing 31 at a level slightly above the flash point thereof, and since the water coil 85 is in intimate contact with these heating elements, it will be seen that as the water passes through this water coil it is not only converted into steam but also is superheated by the time it reaches the outlet bore 88.

From this outlet bore the steam escapes through the perforated cover 90 of the cup-shaped protuberance 89 on top of the casing 31 and hence mixes with the air drawn over the exterior of this casing both to humidify and also preheat this air. Accordingly superheated steam is added to the air drawn up through the air inlet shell 29 and into the inlet 21 of the fan scroll 10. Accordingly it will be seen that in addition to the combustion apparatus being supplied, on each start, with fuel of high volatility, during operation, it is supplied not only with fuel preheated to a lower initial temperature but also with air which is not only preheated but also humidified by the addition of superheated steam. By humidifying with steam instead of water particles, a water vaporair mixture is supplied for combustion and has been found to be more effective in improving combustion and in avoiding carbon deposits. When used in conjunction with intermittent firing internal combustion engines the presence of the water vapor will also control detonation thereby relieving the necessity for using anti-knock compounds. In such use it is also possible to inject the steam under pressure into the cylinders instead of drawing the air in by suction as described.

It will also be seen that with the arrangement shown, the steam is supplied only during on periods of operation of the apparatus but that since the preheater is maintained during off periods of operation at temperature above the flash point of the oil, it effects distillation of the oil as a quiescent body and since the water coil is in intimate contact with the heating element for the oil, the water coil is hot and prepared for the generation of steam on each start.

It will also be noted that full advantage is taken of utilizing normally wasted heat during on period of operation, coils alongside the electrical heating elements being provided for utilizing the waste heat such as by the use of the products of combustion or the jacket water of the apparatus being served.

While a specific embodiment of the application of the invention has been described, it will be appreciated that the invention is subject to wide modification within the limits of the accompanying claims. In particular, it is not essential that the invention be confined to the use of petroleum fuel oils and to space heating, and features of the invention can be employed with all types of fuel and in apparatus where the energy is used for developing power or as a direct propellant. Further, while electrical heating elements have been found desirable in the apparatus as described, it will be apparent that any other means, such as fuel heaters, can be provided for supplying the preheat above described.

I claim:

1. An auxiliary preheater and humidifier for the stream of air supplied to support combustion in combustion apparatus, comprising a first heater maintained at a sulficiently high temperature to generate steam, an air shell surrounding said first heater in spaced relation thereto, a fan arranged to move said stream of air through the space between said first heater and shell in heat exchange relation with said first heater, means arranged to pass a stream of water in heat exchange relation with said first heater to generate steam, means arranged to discharge said steam into said stream of air, a second heater arranged in said space between said first heater and shell and past which said air stream flows, and means to supply a heating medium to said second heater.

2. An auxiliary preheater and humidifier for the stream of air supplied to support combustion in combustion apparatus, comprising a first heater maintained at a sufiiciently high temperature to generate steam, an air shell surrounding said first heater in spaced relation thereto, a fan arranged to move said stream of air through the space between said first heater and shell in heat exchange relation with said heater, means arranged to pass a stream of water in heat exchange relation with said first heater to generate steam, means arranged to discharge said steam into said stream of air, a second heater arranged in said space between said first heater and shell and past which said air stream flows and means arranged to pass a medium heated by the products of combustion in the combustion apparatus through said second heater.

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